Businesses have been buying access to Prime Minister John Key through an Olympic fundraiser.

Duco Events, which is organising the Prime Minister's Olympic Gala Dinner on June 28, would not say how much it would have cost to buy a seat on the PM's table, which sold out quickly, but it did say that next time around the price would be around $30,000.

The buyers of the tickets on the PM's table were ''corporates and very rich people'', said Duco, which advertised the event as ''an outstanding opportunity for you to interact with the nation's powerbrokers including cabinet ministers''.

Green Party MP Holly Walker said the marketing for the event carried a clear message, and it was time to look at how corporates and the wealthy used such events to lobby for causes at the highest level.

''There is an implication that this is your opportunity to get in the ear of these decision makers,'' Walker said. ''What it highlights is the inequality of access that people have to decision makers.''

The prime minister was not the only major politician available.

For those with a more limited wallet, but who wanted cabinet access, there was a smorgasbord of ministers to dine with, including Chris Tremain (internal affairs), Paula Bennett (social development), Jonathan Coleman (state services) and Amy Adams (environment).

While it was relatively easy for ordinary people to get to talk to their local MP, those with money appeared to be able to get better access to ministers and it was time for greater transparency, Walker said, adding the Greens' Lobbying Disclosure Bill dealt with this scenario.

Modelled on Canadian law, the bill, if it becomes law, would not end charitable bashes, which are needed to raise money for important causes, but it would require those who used such events to lobby ministers to reveal their activities on a public register.

Penalties of $10,000 for an individual and $20,000 for a company could be imposed for professional lobbyists who fail to file a return.

The prime minister told the Sunday Star-Times that people from many walks of life had access to him.

''I go to many different events, including charity and fundraising events, including those of the sort you describe, where my presence might be beneficial to a charity or good cause,'' he said.

''That needs to be set against the large number of events I attend every week around New Zealand where people can engage with me without cost. I go to dozens of events every week, from school functions and meetings to sporting and cultural events. I engage with hundreds of people every week on many different issues of concern to them.

''I am proactive in terms of accessibility, and I really enjoy getting out of Wellington and meeting people from all walks of life.''

Walker said the Greens bill would not shut down anyone's access to ministers, who had to be able to talk to a wide range of people about issues in order to be able to make decisions, but the aim of the bill was to ''make it more obvious to the public who is influencing our decision makers''.

She was hopeful the bill would get a first reading in Parliament and be referred to select committee for further discussion and refining.

She said both National and Labour had informally said they would not oppose a first reading of the bill which was originally the work of former Green MP Sue Kedgley.